Former FBI agents say it may take a long time to wash away the stain of political bias accusations lobbed against the bureau, which had long been viewed as the nation’s premier law enforcement agency. The criticism of the storied bureau has been bipartisan, with Republicans pointing to accusations the agency mishandled the justification for spying on a Trump campaign figure. Before that it was Democrats furious at the FBI’s handling of presidential candidate Hillary Clinton’s email failures. In between were a slew of embarrassing text messages from an agent involved in both the Trump and Clinton investigations, revelations of...